The general purpose of the invention is to provide a post tensioning system having an alternate means of encapsulating tendon anchorages that requires fewer parts and less labor to install, and provides superior encapsulation results.
For many years, the design of concrete structures imitated the typical steel design of column, girder and beam. With technological advances in structural concrete, however, its own form began to evolve. Concrete has the advantages of lower cost than steel, of not requiring fireproofing, and of its plasticity, a quality that lends itself to free flowing or boldly massive architectural concepts. On the other hand, structural concrete, though quite capable of carrying almost any compressive (vertical) load, is extremely weak in carrying significant tensile loads. It becomes necessary, therefore, to add steel bars, called reinforcements, to concrete, thus allowing the concrete to carry the compressive forces and the steel to carry the tensile (horizontal) forces.
Structures of reinforced concrete may be constructed with load-bearing walls, but this method does not use the full potentialities of the concrete. The skeleton frame, in which the floors and roofs rest directly on exterior and interior reinforced-concrete columns, has proven to be most economic and popular. Reinforced-concrete framing is seemingly a quite simple form of construction. First, wood or steel forms are constructed in the sizes, positions, and shapes called for by engineering and design requirements. The steel reinforcing is then placed and held in position by wires at its intersections. Devices known as chairs and spacers are used to keep the reinforcing bars apart and raised off the form work. The size and number of the steel bars depends completely upon the imposed loads and the need to transfer these loads evenly throughout the building and down to the foundation. After the reinforcing is set in place, the concrete, a mixture of water, cement, sand, and stone or aggregate, of proportions calculated to produce the required strength, is placed, care being taken to prevent voids or honeycombs.
One of the simplest designs in concrete frames is the beam-and-slab. This system follows ordinary steel design that uses concrete beams that are cast integrally with the floor slabs. The beam-and-slab system is often used in apartment buildings and other structures where the beams are not visually objectionable and can be hidden. The reinforcement is simple and the forms for casting can be utilized over and over for the same shape. The system, therefore, produce an economically viable structure. With the development of flat-slab construction, exposed beams can be eliminated. In this system, reinforcing bars are projected at right angles and in two directions from every column supporting flat slabs spanning twelve or fifteen feet in both directions.
Reinforced concrete reaches its greatest potential when it is used in pre-stressed or post-tensioned members. Spans as great as 100 feet can be attained in members as deep as three feet for roof loads. The basic principal is simple. In pre-stressing, reinforcing rods of high tensile strength wires are stretched to a certain determined limit and then high-strength concrete is placed around them. When the concrete has set, it holds the steel in a tight grip, preventing slippage or sagging. Post-tensioning follows the same principal, but the reinforcing is held loosely in place while the concrete is placed around it. The reinforcing is then stretched by hydraulic jacks and securely anchored into place. Pre-stressing is done with individual members in the shop and post-tensioning as part of the structure on the site.
In a typical tendon tensioning anchor assembly in such post-tensioning operations, there is provided a pair of anchors for anchoring the ends of the tendons suspended therebetween. In the course of installing the tendon tensioning anchor assembly in a concrete structure, a hydraulic jack or the like is releasably attached to one of the exposed ends of the tendon for applying a predetermined amount of tension to the tendon. When the desired amount of tension is applied to the tendon, wedges, threaded nuts, or the like, are used to capture the tendon and, as the jack is removed from the tendon, to prevent its relaxation and hold it in its stressed condition.
Metallic components within concrete structures may become exposed to many corrosive elements, such as de-icing chemicals, sea water, brackish water, or spray from these sources, as well as salt water. If this occurs, and the exposed portions of the anchor suffer corrosion, then the anchor may become weakened due to this corrosion. The deterioration of the anchor can cause the tendons to slip, thereby losing the compressive effects on the structure, or the anchor can fracture. In addition, the large volume of byproducts from the corrosive reaction is often sufficient to fracture the surrounding structure. These elements and problems can be sufficient so as to cause a premature failure of the post-tensioning system and a deterioration of the structure.
There are four general types of prior art systems for protecting post-tensioning anchor systems from deterioration and failure:
(1) Rodriguez (U.S. Pat. No. 4,821,474) teaches an anchor plate assembly having collar regions on both sides for the attachment of caps or tubular members to cover and seal the tendon anchored within the anchor plate. On the side of the anchor plate intended to face the concrete the collar region has a smaller outside diameter than the collar region of the side facing the concrete form. On the side of the anchor facing the concrete form the larger collar region has an inner wall and an outer wall region. An annular groove is formed between the inner and outer walls that is adapted to receive the tubular member or cap. This system has the disadvantage of requiring the machining of the annular groove in the larger collar region, which increases the cost and complexity of Rodriguez's anchor plate. Rodriguez also calls for optional securing filaments in the form of wires or plastic straps, which secure the tubular member and cap to the anchor plate. Connecting ears formed on opposite sides of tubular member receive the optional filaments therearound. In the practice of the design taught by Rodriguez, the securing filaments are not optional, but are required to prevent the tubular members and caps from falling off the anchor plate during use. This is disadvantageous because it requires additional parts and labor to install the post tensioning anchors. PA1 (2) The system described in Reinhardt (U.S. Pat. No. 4,773,198) utilizes an anchor plate with threads machined into the inside face of a collar for receiving a threaded cap. The Reinhardt anchor plate also has a differently shaped collar on the opposite side of the anchor plate for receiving a connector. This system has the disadvantage that different attachments must be used on opposite sides of the anchor plate, thus, increasing the difficulty of installing the system and adding to the cost of the system. This system has the further disadvantage of requiring machine tooled threads on both the anchor plate and the cap, which add to the cost of the system. A still further disadvantage of the system is that the threads on the cap and anchor can be stripped during installation, thus, preventing proper protection for the tendon anchorage. PA1 (3) Another system, sold by VSL Corporation (Campbell, Calif.), uses snap-on caps to cover the tensioning tendon on the wedge side of the anchor plate. This system operates by means of a flange on the cap, which snaps into a groove on the inside of the collar. This system also requires the use of two different attachments to fit different sized collars on opposite sides of the anchor plate. The use of a snap on cap has the further disadvantages of requiring that the anchor plate be machined to include the annular groove inside the collar into which the flange of the cap fits. PA1 (4) Other systems taught by Sorkin (U.S. Pat. No. 4,896,470) and Davis et al. (U.S. Pat. No. 4,616,458) require the complete coverage of the anchor plate with plastic. Sorkin calls for encapsulation by molding plastic around the anchor plate with a cap being inserted inside a collar region on the wedge side of the anchor plate. In the Davis et al. system, a molded plastic top member for covering the entire front face of the anchor plate snap fits onto a molded plastic bottom member that covers the entire rear face of the anchor plate. The Davis et al. system requires extensive manipulation of the top and bottom members to make them fit securely around the anchor plate and has the disadvantage of increased labor costs. Both of the above-described encapsulation systems have the disadvantage that the wedge faces thereof are subject to melting when excess tendon is cut off with a torch, which can cause failure of the encapsulation.
With the exception of Rodriguez, all of the prior art systems have the disadvantage of having the cap attached inside the collar, which can permit water, carrying corrosive compounds, to enter the collar and seep into contact with the tendon and wedges. All of the prior art systems have the further practical and cost disadvantages of requiring different parts for attachment to opposite sides of the anchor plate.